1. Field of Invention
The invention disclosed herein relates to propeller improvement, particularly to noise and cavitation abatement on propellers used on marine propulsion, fluid pumps, fans, and power turbines. Noise is always undesirable collateral of machine function, is source of illness, operational restrictions, and service life shortened on devices affected by noise. It is also a security concern to deal with when discrete operation is required.
2. Discussion of the Prior Art
Propellers accelerate a fluid by impinging it against a rotating blade. Such acceleration provides the thrust to an object to which it is connected. A common cause of noise is created by the turbulence of the fluid, cavitation and the vibration of the propellers' blades. Some designs achieve lower noise by means of a hardened blade structure, or surrounding the entire propeller with a duct. Other designs also reinforce the blade structure linking all blades to a ring at the propeller perimeter. Other designs use screw-type blade propellers.
Turbulence is often caused by the blade itself. The flat shape of a blade generates parasite waves that generate turbulence. This turbulence generates noise when the fluid knocks against the blade's surface or other structures near the propeller. This fluid impact creates structural tension that when released generates sound waves. To reduce the noise caused by the structural tension, reinforcements to the propeller blade structures are introduced. Some reinforcement strategies use a greater blade section or use reinforced materials on blade structures. Others use a ring attached at the extremes of the blades surrounding the propeller (see U.S. Pat. Nos. 1,441,852; 1,518,501 and 4,684,324 and their citations).
The basic technique of laminar turbulence reduction on the blades has been to design screw blade propellers as on U.S. Pat. No. 1,518,501. The blades described therein have an open semicircular chamber, which shapes help to avoid turbulence generated by flat shapes and also helps to reduce the angular momentum perturbation on the particles.
The Inertial cavitation phenomena is caused when the fluid pressure falls below it's vapor pressure due high acceleration momentum. The cavitation causes shock waves that are a powerful noise source. Prior art systems are designed to reduce cavitation by using larger diameter propellers to move a bigger volume (and mass) of fluid at a lower acceleration. Other systems inject gas under pressure. As expected, this avoids the shock waves but introduces a new source of noise to the system due the turbulence generate by such gas (see U.S. Pat. No. 4,188,906 and its citations).
All these solutions actually improved the noise abatement on the prior art systems, but didn't deal with the root of the problem. The turbulence generated by the particle circular momentum against the blade surface. Such turbulence causes a noise whose frequency is proportional to the rotational speed of the propeller.